1. Field of the Invention
The invention is generally related to the area of optical components and devices for optical communications. In particular, the present invention is related to techniques for packaging optical components and devices in compact size and more economic way without compromising overall performance of such components and devices.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high capacity capabilities. Wavelength division multiplexing (WDM) is an exemplary technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength. Using the WDM system, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. To take the benefits and advantages offered by the WDM system, there require many sophisticated optical network elements.
Optical add/drop and multiplexer/demultiplexer devices are those elements often used in optical systems and networks. For example, an exchanging of data signals involves the exchanging of matching wavelengths from two different sources within an optical network. In other words, an add/drop device can be advantageously used for the multi-channel signal for dropping a wavelength while simultaneously adding a channel with a matching wavelength at the same network node. Likewise, for transmission through a single fiber, a plurality of channel signals are combined via a multiplexer to be a multiplexed signal that eventually will be separated or demultiplexed via a demultiplexer.
A fundamental element in an add/drop device or a multiplexer/demultiplexer is what is called a three-port device. As the name suggests, a three-port device has three ports, each for a multi-channel signal, a dropped or added signal or a multi-channel signal without the dropped or added signal. FIG. 1A shows a typical design of a three-port add/drop device 200. The optical device 200 includes a common (C) port 202, a reflection (R) port 204, and a transmission (T) port 206. When the device 200 is used as a multiplexer (i.e., to add a signal at a selected wavelength λK to other signals at wavelengths other than the selected wavelength λK), the T-port 206 receives a light beam at the selected wavelength λK that is to be multiplexed into a group of beams at wavelengths λ1, λ2, . . . λN excluding the selected wavelength λK coupled in from the C-port 202. The R-port 204 subsequently produces a multiplexed signal including all wavelengths λ1, λ2, . . . λK, . . . λN.
Likewise, when the optical device 200 is used to demultiplex signals, the C-port 202 receives a group of signals with wavelengths λ1, λ2, . . . λK, . . . λN. The T-port 206 produces a signal with the selected wavelength λK while the R-port 204 subsequently produces a group of signals including all wavelengths λ1, λ2, . . . λN except for the selected wavelength λK. In general, the optical paths towards a R-port and a T-port are respectively referred to as R-channel and T-channel.
FIG. 1B shows an exemplary internal configuration 210 of the optical device 200 of FIG. 1A. As shown in FIG. 1B, there is a first GRIN lens 212, an optical filter 214 (e.g., a multi-layer thin film filter) and a second GRIN lens 216. In general, a dual-fiber pigtail is provided in a holder 218 (e.g., a dual-fiber pigtail collimator) and coupled to or positioned towards the first GRIN lens 212, and a single-fiber pigtail is provided in a second holder 220 and coupled to or positioned towards the second GRIN lens 216. Essentially the two GRIN lenses 212 and 216 accomplish the collimating means for coupling an optical signal with multi channels or wavelengths in and out of the C port 202, the R port 204, or the T port 206. In general, the three-port device 200 is known to have a very low coupling loss from the C-port to both the R-port and the T-port for use as a demultiplexing device, or vise versa as a multiplexing device.
The performance, reliability and cost of such devices are primarily controlled by the designs and packaging technologies thereof. U.S. Pat. No. 6,282,339 has reviewed two kinds of design and packaging technologies of wavelength division multiplexed (WDM) couplers and lists their respective problems. Accordingly, U.S. Pat. No. 6,282,339 discloses an improved design and process for fabricating a low-cost WDM coupler with improved reliability by preventing the epoxies to spread over or diffused into the optical paths in the coupler. By eliminating the epoxies from all the optical paths, the difficulties and limitations in the two kinds of design and packaging technologies can be fully overcome.
FIG. 2A and FIG. 2B respectively duplicate FIG. 2A and FIG. 2B of U.S. Pat. No. 6,282,339. In FIG. 2A, a WDM filter 105 is attached to a first GRIN lens 110 by applying a first heat-curing epoxy 115. To prevent the epoxy from entering the optical path between the WDM filter and the GRIN lens, an EP42HT heat-curing epoxy is employed as the first heat-curing epoxy 115. The EP42HT epoxy has good resistance to high temperature and humidity. For the purpose of completely preventing the EP42HT epoxy from entering the optical path between the WDM filter and the GRIN lens in any situation, a special bonding and curing process is developed. The EP42HT epoxy is first prepared and then gelled at room temperature for about two hours. Then the EP42HT epoxy is applied to the boundary areas between the WDM filter and the GRIN lens. Then the EP42HT epoxy is further gelled at room temperature for another one hour. Finally, the EP42HT epoxy is cured at 110C for another one hour. According to this manufacturing process, the EP42HT epoxy will enter only the very outside interface areas between the WDM filter and the GRIN lens to provide bonding between the WDM filter and the GRIN lens but not the optical path between the WDM filter and the GRIN lens. To obtain strong bonding between the WDM filter and the GRIN lens, a certain amount of the EP42HT epoxy is applied around the interface area between the WDM filter and the GRIN lens. Usually, the diameter of the EP42HT epoxy bonding will be slightly larger than that of the GRIN lens.
After the WDM filter 105 is attached to the first GRIN lens 110, the first GRIN lens 110 attached to the WDM filter 105 is inserted into a first holding tube 120. The first holding tube 120 has a length slightly longer than the combined length of the WDM filter 105 and the first GRIN lens 110. The first GRIN lens 110 and the WDM filter 105 is fixed to the first holding tube 120 by applying a second heat curing epoxy 125. Since the diameter of the epoxy bonding 115 is slightly larger than that of the first GRIN lens 110, the holding tube 120 has larger inside diameter at the side of the epoxy bonding 115 than at the other side. After a second holding tube 130 is mounted onto a dual fiber pigtail 135, the filter 105, the lens 110, and the tube 120 sub-assembly and the fiber pigtail 135 with the holding tube 130 are mounted on an alignment stage (not shown). Then a distance and orientation of the fiber pigtail 135 relative to the GRIN lens 110 is adjusted to achieve a lowest reflection loss from the input fiber 140 to the output fiber 145. After the fiber pigtail 135 is placed at its optimal position relative to the GRIN lens 110, the position of the holding tube 130 is adjusted so that its end surface is in contact with that of the holding tube 120. Then a third heat-curing epoxy 150 is applied to fix the fiber pigtail 135 and the two holding tubes 120 and 130 together and thus an assembly of a dual fiber collimator 155 is completed. After the epoxy 150 is applied, it will spread over all contact areas between the holding tube 130 and the fiber pigtail 135 and between the holding tubes 120 and 130. However, it will not contaminate the optical path between the GRIN lens 110 and the fiber pigtail 135 because of surface tension.
In FIG. 2B, a second GRIN lens 160 is inserted and fixed into a third holding tube 165 having a length slightly longer than that of the GRIN lens 160 by applying a fourth heat-curing epoxy 170. Then a single fiber pigtail 175 is inserted into a fourth holding tube 180. The single fiber pigtail 175 is a low-cost standard single fiber pigtail, like that used in the pending patent application. Then the first holding tube 120 with the first GRIN lens 110 and the WDM filter 105, the third holding tube 165 with the second GRIN lens 160 and the fourth holding tube 180 with the single fiber pigtail 175 are mounted on an alignment stage (not shown). A pigtail position-adjustment is made on the alignment stage to achieve optimal positions of the first GRIN lens 110 to the second GRIN lens 160 and the single fiber pigtail 175 to the second GRIN lens 160 with a lowest transmission loss.
However, it has been noticed that the approach disclosed in U.S. Pat. No. 6,282,339 demonstrate at least the following problems that can, in return, complicate the designs and introduce costs in packaging an optical device (e.g., a WDW coupler). First of all, the device 100 in FIG. 2A or FIG. 2B requires that the first holding tube 120 (i.e., a glass tube) that packages the WDM filter 105 and the GRIN lens 110 has to be made with two distinctive cross-sections (e.g., different diameters). A smaller inner diameter section is to match to that of the GRIN lens 110 (e.g., 1.8 mm diameter) and the larger inner diameter section is to accommodate the WDM filter 105 that is typically in a square cross-section shape. For a variety of applications, such a filter may have to be in different square sizes due to coating stress concerns. Some of the most popular sizes used in the industry are 1.4×1.4 mm, 1.6×1.6 mm, 1.9×1.9 mm. Thus the attached GRIN lens and filter combination can have various different cross-sections (e.g., ˜1.98 mm, ˜2.26 mm, and ˜2.69 mm, respectively, if the popular sizes are used). Making these types of glass tubes in large quantity will inevitably become a costly problem since the most inexpensive method to make a glass tubing is through extrusion. Finally, tube segments are formed by cutting and end polishing.
Another problem is the stress-induced power loss that may experienced in the device 100 in FIG. 2A or FIG. 2B. The bonding of the GRIN lens 110 to the holding tube 120 (so is the bonding of the GRIN lens 160 to the holding tube 165) is done by thermally cured epoxy. The use of the thermally cured epoxy takes a substantial time and such epoxy typically puts some stress on materials being bonded. However, the stress can slightly change the optical property of the GRIN lens, thereby inducing an undesirable optical power loss.
Accordingly, there is a great need for techniques for providing optical couplers of consistent performance, reliability and low cost. Such devices so designed are amenable to small footprint, broad operating wavelength range, enhanced impact performance, lower cost packaging, and easier manufacturing process.